1. Field of the Disclosure
The present disclosure relates to a burst optical signal transmission device which transmits a burst optical signal and the like its a control method.
2. Discussion of the Background Art
Recently, along with rapid popularization of the Internet, a large capacity, advanced, and economical optical access system has been required. Research of a passive optical network (PON) is underway as a method of realizing such a system. The PON is an economically advantageous optical access communication system in which a plurality of users can share a transmission path between a center device and an optical passive element by concentrating a plurality of transmission paths from a plurality of users to a unique transmission path by the optical passive element such as an optical power splitter. In Japan, an economical optical access communication system Gigabit Ethernet (TM)-PON (GE-PON) in which up to 32 users share 1 Gb/s class line capacity by time division multiplexing (TDM) is currently introduced. Research of a 10 Gb/s class 10G-EPON is underway as the next-generation optical access system capable of responding to needs of further large capacity, the system capable of realizing large capacity by an increased bit rate of an optical transmitter/receiver while using the part of transmission path which is the same path of the existing GE-PON.
FIG. 1 illustrates a configuration of a conventional burst optical signal transmission device for PON focusing only on a transmitting unit of a transceiver 1 mounted on an ONU (receiver unit and other peripheral circuits are not illustrated). FIG. 2 illustrates a time chart of a burst signal control method of the conventional burst optical signal transmission device for PON illustrated in FIG. 1. In FIG. 1, the conventional optical transmitter is mainly formed of a DML transmitter optical subassembly 2 (DML-TOSA) including a direct modulation LD 3 (DML: transmitter optical subassembly) such as a Fabry-Perot LD (FP-LD: laser diode) and a distributed-feedback LD (DFB-LD) and a burst supporting LD driving circuit 4. Burst signal light 0 transmitted from the transceiver 1 is generated in the following manner. Transmission signal data 5 transmitted from an upper layer (not illustrated) of the transceiver 1 is formed of an idle signal 52 and a data signal 51 (refer to FIG. 2). A burst control signal 6 also transmitted from the upper layer (not illustrated) generates the burst signal light by controlling on/off of the burst supporting LD driving circuit 4 according to transmission allowed time allocated to the ONU. Values of bias current (Ib) 61 and modulation current amplitude (Ipp) 62 supplied from the burst supporting LD driving circuit 4 to the DML-TOSA 2 through an LD signal line 7 are set according to on/off of the burst control signal 6. Meanwhile, the bias current 61 being a constant current value is 30 mA/0 mA at the time of on/off respectively in the conventional example illustrated in FIG. 2. The modulation current amplitude 62 being a current amplitude value on which high-speed intensity modulation at 1 Gb/s is performed is 40 mA/0 mA at the time of on/off respectively in the conventional example illustrated in FIG. 2. Thus, when the burst control signal 6 is turned off, LD driving current (bias current 61 and modulation current amplitude 62) is 0 mA and the optical signal is not transmitted. When the burst control signal 6 is turned on, the signal light on which intensity modulation at 1 Gb/s is performed is transmitted within a range from 10 mA to 50 mA according to the data signal 51 around the bias current 61 of 30 mA. In this manner, in the conventional burst optical signal transmission device for PON, it is possible to transmit the burst signal light only by controlling the LD driving current. It is required that the ONU does not transmit the optical signal or that optical intensity is not higher than a certain reference value outside the transmission allowed time in PON specification. The reference value is specified to be “lower than (minimum reception sensitivity of OLT receiver—10 dB)” (lower than approximately −40 dBm) in ITU-T standard G-PON and XG-PON1 and specified to be lower than −45 dBm in IEEE standard GE-PON and 10G-EPON, for example. The LD driving current can be set to 0 mA outside the transmission allowed time in the conventional burst optical signal transmission device for PON, so that this may sufficiently satisfy the specification.
In contrast, large capacity over 10 Gb/s class might be required depending on a service such as a high-definition image service, a higher bit rate of the transmitter/receiver (40 or 100 Gb/s class) causes a sharp increase in cost of the transmitter/receiver, so that there is a problem that a practical system cannot be obtained. A wavelength-variable WDM/TDM-PON obtained by combining the TDM and wavelength division multiplexing (WDM) in which wavelength variability is added to the optical transmitter/receiver such that the optical transmitters/receivers in a station side device can be gradually increased according to band requirement is reported as means of realizing economical large capacity (refer to Non Patent Literature 1, for example).    Non Patent Literature 1: S Kimura, “WDM/TDM-PON Technologies for Future Flexible Optical Access Networks”, 15th OECC 2010, 6A1-1.